The Rotary Drum Precoat Filter

Click on the
thumbnailto maximize the image

Description

The Rotary Drum Precoat Filter is used to polish solutions having traces of
contaminating insolubles. To polish the solution the drum deck is precoated with
a medium of a known permeability and particle size that retains the fines and
produces a clear filtrate.

Cellulose consisting of fibrous light weight and ash less paper like
medium.

Special ground wood is becoming popular in recent years since it is
combustible and reduces the high cost of disposal. There are nowadays
manufacturers that grind, wash and classify special timber to permeabilities
which can suit a wide range of applications.

These materials when related to precoating are wrongly called filter-aids
since they do not aid filtration but serve as a filter medium in an analogy to
the filter cloth on a conventional drum filter.

The Precoat Filter is similar in appearance to a conventional
Drum Filter but its construction is very different:

The scraper blade on conventional drum filters is
stationary and serves mainly to deflect the cake while it is back-blown at
the point of discharge. The scraper on a precoat filter, which is also
called "Doctor Blade", moves slowly towards the drum and shaves-off the
blinding layer of the contaminants together with a thin layer of the
precoating material. This movement exposes continuously a fresh layer of the
precoat surface so that when the drum submerges into the tank it is ready to
polish the solution. The blade movement mechanism is equipped with a
precision drive having an adjustable advance rate of 2-10 mm/hr. The
selected rate is determined by the penetration of fines into the precoat bed
which, in turn, depends on the permeability of the filter aid. Once the
entire precoat is consumed the blade retracts at a fast rate so that the
filter is ready for a new precoating cycle.

The cake discharges on conventional drum filters by
blow-back hence a section of the main valve's bridge setting is allocated
for this purpose. On precoat filters the entire drum deck is subjected to
vacuum therefore there are two design options:

A conventional valve that is piped, including its
blow-back section, to be open to vacuum during polishing. When the
precoat is consumed its blow-back section is turned on to remove the
remaining precoat heel over the doctor blade.

A valveless design in which there is no bridge
setting and the sealing between the rotating drum and the stationary
outlet is by circumferential "o" rings rather than by a face seal used
on conventional valves.

The clearance between the drum deck and the agitator on
conventional drum filters is sufficient to accommodate cakes of up to about
50 mm while on modern precoat filters higher clearances to allow the
formation of 150 mm precoats are quite common.

A typical flow scheme of a
Precoat Filter Station

The main components of a
Precoat Filter

The Drum

The drum deck is divided into separately isolated compartments each
subjected to vacuum while the drum is in rotation. The
compartments are divided with grooved division strips along the drum face
and around the circumference of the drum heads. The grids cover the entire drum
surface and serve to
support the filter cloth. The filter cloth itself is fastened to the drum
by inserting special caulking ropes into the grooves.

Since the water flow during precoating is very high the grids are
of special deep design and the internal pipes are larger in diameter
than the conventional piping to reduce head loss.

The Conventional
Valve Design

The conventional
valve is basically similar to the Drum Filter valve shown on the right.

It has three connections and a bridge
setting that separates between the cycle zones:

The bottom
connection for the precoat bed
formation during the precoating and polishing stages.

The upper connection for the
precoat bed drying during precoating and polishing stages.

The one below the cake drying section
is not used during the polishing stage and is turned on just
to blow away the remaining precoat at the end of the polishing
cycle.

The
Valveless Design

The valveless filter is a
relatively newcomer to the range of Precoat Filters and is based on
the concept that no bridge setting is required since the drum face
is under constant vacuum throughout the clarification cycle. This
design is much cheaper than the conventional design since the
internal piping is very simple
as compared to the complex internal piping on conventional filters.

The procedure how to start
the precoating stage on a conventional filter is described in detail
in the section Precoating below.

To start the precoating on a
valveless filter it is required to build a thin layer of
diatomite or perlite that will seal the drum's upper surface that is
exposed to atmosphere. This will create a low vacuum on the entire
drum surface and will start to formation of the precoat bed.

The Internal Piping

As may be seen on the picture to the right the
internal piping on a valveless precoat drum runs straight from the
drum face towards the center manifold that serves also as the drum's
main shaft.

This, when compared to the conventional drum
piping has several advantages:

Simple construction and easy access in case
of leakages.

Large diameter piping that reduces head loss
mainly during the precoating stage when high rates of water flow
through the piping.

The Agitator

An agitator keeps gently the precoat slurry in suspension
during precoating and reciprocates between the drum face and tank bottom
at 16 CPM.

The Doctor Blade

The surface of the precoat bed is very dense and the degree of polishing
depends on the permeability of the various grades of the diatomite or
perlite . The layer of the contaminating substances that is retained on the
bed surface is always very thin so special doctor blades have been developed over the
years with very slow moving blades.

The latest nano technologies introduced variable frequency motors that
can reduce the rotation speed down to 0.4 Hz without losing torque. Such motors,
generally 1 kW with VFD, when coupled with
high accuracy gearboxes move the doctor blade at a speed of
1.8-18 mm/hr so the cutting edge "shaves off" a very thin layer of the
contaminating substances and saves unnecessary consumption of diatomite.
Once the precoat at the end of the cycle is consumed the blade retracts at a
speed of 10-12 minutes and the filter is then ready for new precoating.

Due to the abrasiveness of the dense diatomite precoat
the cutting edge of doctor blade is divided to short segments to allow the
sharpening of worn segments. The segments are generally coated with a wear resistant stellite alloy.

The Startup Procedure

To startup a valveless
filter there is one major requirement:

To allow very fast drum
speed that will pick up a thin layer of diatomite or perlite
in the first minute of the precoating stage. Once a vacuum is created
the speed can be reduced gradually to form a dense and compact precoat
bed.

Selection Criteria

Precoat filters are generally used in the following cases:

Solutions that require a very high degree of clarity.

The solids content is very low and less than 2-3%.

The solids are sticky and would otherwise clog the filter cloth.

Operational Sequence

The operation of a precoat filter is done in two stages:

Precoating stage

Polishing stage

The two stages are closely related since the efficient and economical
performance of the filter in the polishing stage depends entirely on the
selection of a suitable filter aid and the care taken during the precoating
stage.

A 2-5% precoat slurry is prepared by feeding the selected filter aid to a
tank filled with water and equipped with a low speed mixer, such as a sweep-arm
agitator, that rotates at 30-50 rpm. The low speed is necessary in order to
avoid the breakage of the fine skeletal or expanded rock structure.

A precoat cake, depending on its permeability, may contain about 50% moisture
so it is sometime advisable to prepare the slurry with polished filtrate rather
than with water. Two arguments exist in favor of the filtrate option:

The amount of dry diatomite in the wet cake is 310 Kg/m3 (for coarse
diatomite) and 370 Kg/m3 (for fine diatomite) so, if we assume a 150 mm
precoat then it may contain some 50 Kg/m2 water. Therefore, on a middle sized
filter of 20 m2 the amount of water for downstream evaporation is 1 ton and
this must be taken in consideration.

With filtrate the transition from precoating to polishing does not
necessitate the emptying of the tank whilst with water the tank must be
drained in order not to dilute the solution.

When the alternative to water is preferred attention must be given to the
specific gravity of the filtrate since difficulty may arise while attempting to
wet and mix the light filter aid in a solution of say 52% CaBr2 having a
specific gravity of 1.7. In such a case the solution of using a high speed mixer
is not desirable since, as mentioned above, particle breakage may occur so using
an eductor to enhance wetting is a better method .

There are some basic rules to observe during the precoating of the drum in
order to ensure that the filter aid is evenly distributed and the formed bed is
tight and crack free:

When precoating commences the cake resistance is low therefore the vacuum
pump should be capable to deliver a very high air flow and in some
installations a separate vacuum pump is dedicated just for the precoating
stage.

The precoat filtrate pump should be capable of pumping the slurry at a
pipe velocity of 1.5-2.0 m/sec to avoid segregation and the settling of the
heavier filter aid particles.

For the same reason the drum should be rotated at a high speed.

The weir in the overflow box should be lowered so that the drum is barely
submerged and the bed does not form too fast. A fast forming precoat is
generally uneven in thickness and requires substantial "shaving" prior to
feeding the contaminated solution in the polishing stage. Likewise, a precoat
that forms too fast tends to crack so filtrate clarity may deteriorate and
the cloth under such cracks may blind.

The doctor blade must be retracted to the maximum clearance between drum
deck and agitator and then advanced slowly to smoothen the bed's surface in
preparation for the polishing stage.

Vacuum should never be cut-off during precoating (or polishing for that
matter) since the heavy cake will most certainly fall off the drum into the
tank with all the consequences. A rule of thumb suggests that to hold a 100
mm wet precoat to the drum a vacuum of 180-200 mmHg is required.

The filter cloth acts merely as a support for the precoat that forms when
precoating commences and then the already formed thin layer takes over.
Therefore, the cloth should be sufficiently dense to retain the forming bed
depending to a large extent on the particle distribution of the selected
filter aid.

Polishing

The insoluble contaminants, which are often slimy and gelatinous in nature,
cause fast media blinding and soon reduce filtrate rate to a trickle. Hence, the
operating principle of polishing is to avoid formation of a thick layer but
rather form a very thin layer that is removed continuously by shaving it off.

This requires careful optimization since polishing is an expensive operation.
The optimum conditions are:

Select a precoat material that is permeable enough to yield a high
filtrate rate but dense enough to avoid the penetration of the contaminants
into the precoat bed.

Set the Doctor Blade to the lowest advance speed so that it barely cuts
into the precoat bed since deep penetration of the insoluble matter increases
diatomite consumption and shortens the polishing cycle.

Maintenance

All the points mentioned in the section related to conventional
D:\Application Data\Downloads\valvless internal piping.jpg also apply to Precoat Filters but
some additional requirements should be observed:

Diatomaceous earth or perlite are abrasive materials and the Doctor
Blade's edge must be kept sharp otherwise the cut is not smooth and
vibrations to the fine discharge mechanism may be experienced. A ground and
polished seam of stellite is often welded along the edge to prolong the life
of the blade.

Good practice is to manufacture the blade from short segments since
practice shows that the wear is not equal across the drum's face. Preferably,
these segments should be manufactured with two sharp edges so that when one
edge wears out it may be turned 180 degrees thus doubling the time required
for replacement.

The moving parts of a rake type reciprocating agitator have many bearings
that once they wear out the entire filter is subjected to shocks and shakes.
This situation is dangerous since during the precoating stage the vacuum is
relatively low and the forming bed may drop-off so special precautions should
be taken to inspect the bearings as part of the preventive maintenance
schedule.